3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB. A UE may more generally be referred to as a wireless device or a wireless terminal.
FIG. 1 illustrates a part of an LTE system. In the Radio Access Network (RAN) an eNodeB 101 serves a UE 103 located within the eNodeB's geographical area of service or the cell 111. The eNodeB 101 is connected via an X2 interface to a neighboring eNodeB 102 serving another cell 112. The two eNodeBs 101 and 102 are connected to a core network node called Mobility Management Entity (MME) 104. The core network in LTE is sometimes referred to as Evolved Packet Core (EPC), and the MME is one of the core network nodes in EPC. Together, the E-UTRAN, the EPC and potentially other entities too, such as service related entities, are referred to as the Evolved Packet System (EPS). S1 Application Protocol (AP) provides the signaling service between E-UTRAN and EPC.
A homogeneous network is a RAN comprising network nodes, such as RBS, eNodeB, Remote Radio Heads (RRH), and Remote Radio Units (RRU), in a planned layout. In the homogeneous network all network nodes have similar transmit power levels, antenna patterns, and receiver noise floors, as well as similar backhaul connectivity to a data network. A Heterogeneous Network (HetNet) is a RAN comprising several different types of network nodes serving the cells. The types of network nodes are different with respect to, for example, transmission power, radio bandwidth, backhaul capacity, and placement. These different types of network nodes interact to provide network access and communication services to a set of wireless terminals or UEs. In one example of a HetNet, low power nodes such as micro, pico, femto, or relay base stations are deployed in addition to a planned or regular placement of high power nodes such as wide area RBSs serving macro cells. Such low power nodes are often deployed to eliminate coverage holes in the homogeneous network and to improve capacity in hot-spots. Due to their lower transmit power and smaller physical size, low power nodes can offer flexible site acquisitions.
In HetNets, the traditional mechanism used to allocate UEs to cells, based on a relative Signal to Interference plus Noise (SINR) for candidate cells at the UE's location, is insufficient. Cells served by low power nodes are expected to off-load the cells served by high power nodes for relatively stationary UEs with a high bandwidth demand. However, their transmission power is generally not sufficient to dominate in terms of relative SINR over the signals transmitted by adjacent high power nodes with higher transmit power. This may be true even for UEs that are quite close to the low power node.
The prevalent solution for this problem in currently deployed systems is to manually configure a range expansion offset parameter based on an expected network load in a given area. This may be feasible for situations where load, node placement, and interference are fairly static. However, this will not be the case in many future scenarios. Low power nodes may e.g. be added without much planning or network operator control over exact placement and UE traffic demand. Furthermore, UE mobility may vary widely on both shorter and longer time scales. Therefore, manually configuring e.g. range expansion offset parameters of networks in such scenarios may not be a viable alternative.
Load balancing for cellular networks has been fairly well studied, where the general idea to base balancing on measurements of the current load distribution in the network is known. In “I. Siomina and Di Yuan, Load balancing in heterogeneous LTE: Range optimization via cell offset and load-coupling characterization. In Communications (ICC), 2012 IEEE International Conference, pages 1357-1361, June 2012” a method is described which is based on integer programming to assign offset values to each node, given load levels of the entire network. A drawback of the method is that it needs to be centralized and requires collecting and transferring load estimates to a central location. A time-consuming optimization mechanism is then used to determine suitable values for the offset parameter, which only then can be redistributed to the nodes of the network. It is unclear how the delays and scalability issues implied by such a mechanism should be handled. Similar issues arise in an approach described in “Hao Wang, Lianghui Ding, Ping Wu, Zhiwen Pan, Nan Liu, and Xiaohu You; Dynamic load balancing and throughput optimization in 3gpp LTE networks; In Proceedings of the 6th International Wireless Communications and Mobile Computing Conference, IWCMC '10, pages 939-943, New York, N.Y., USA, 2010; ACM”. Also this approach is centralized. It uses enforced handovers rather than adapting range expansion offset parameters.
The proposal described in “P. Fotiadis, M. Polignano, D. Laselva, B. Vejlgaard, P. Mogensen, R. Irmer, and N. Scully. Multi-layer mobility load balancing in a heterogeneous LTE network. In Vehicular Technology Conference (VTC Fall), 2012 IEEE, pages 1-5, September 2012” uses an estimate of the remaining available capacity of each node to assign offset values for pairs of nodes based on interactions between eNodeBs on the X2 interface, specifically the S1 TNL Load Indicator and the Composite Available Capacity (CAC) messages. The load indicator is very coarse comprising only two bits. The load indicator is in the proposal only used to determine which nodes should participate in the balancing negotiations. Locally determined CAC values are calculated using a fixed target load value for each node. Pairwise offset values are then computed by scaling CAC ratios with operator specific parameters. Using fixed target load implies an imperfect adaption to variations in load distributions. Furthermore, a separate heuristic is employed to determine when and for which nodes the proposed mechanism should be triggered. The simulations described in the disclosure does not model UE mobility and use only constant UE traffic demands, and does thus not take realistic traffic variations or UE mobility patterns into account.